The invention relates to an attenuator. More specifically, the invention relates to a variable step attenuator which uses devices, such as field effect transistors (FETs), operating in the passive mode to minimize control power depletion and broaden the useful range of band widths of signals which can be attenuated. The invention is especially useful in satellite transponders as well as phased array satellite antennas and phased array radar devices.
There are many applications for attenuators. These applications include not only communications satellites, but also other communications systems, measurement instrumentation, and electrical apparatus such as televisions and radios.
For example in a satellite, especially one used for communications, a frequency division and/or time division multiplex signal transmitted from the ground to a receiving antenna on the satellite is then transmitted to a particular coverage area, such as the continental United States. During transmission of the signal from the ground to the satellite, and from the satellite to the ground there are losses in the amplitude of the signal. Losses in amplitude can also occur while the signal is being processed by circuitry on-board the satellite. Therefore, the signal received by the satellite is amplified by a gain sufficient to ensure that the signal transmitted to the ground will be accurately detected. However, on a satellite, it is important to minimize power consumption. During the life of the satellite its power output for amplifying the signal may decrease. If the gain is set to the largest amount allocated for the gain the set amount of gain cannot be maintained over the life of the satellite because the power output will decrease during the satellite's life. To avoid variations in gain, i.e. to keep gain constant, the gain is intentionally attenuated during an initial period of the satellite's life. Over the satellite's life, the attenuation is decreased so that the gain can be maintained relatively constant. Variable attenuators can also be used in phased array radar systems to adjust the amplitude of radar beam elements, and phased array antennas on-board satellites to control beam shape.
The type of attenuators which have been used include PIN-diode and ferrite step attenuators as well as dual-gate FETs. However ferrite attenuators are very heavy and bulky. PIN-diode devices are operated in active mode, i.e.. they consume power and are relatively susceptible to degradation and failure with the passage of time. The dual-gate FET devices suffer from many of the same drawbacks as the PIN-diode devices. Moreover, dual-gate FETs introduce undesirable phase changes when the attenuation level is varied. In analog devices, a digital to analog converter must be used. There is thus additional power consumption in controlling the device and the quality of operation varies with temperature changes.
As previously noted, present attenuators generally use active devices with analog gate control which requires converters and depletes power.